Pigments are widely used in various applications such as paints, inks, plastics, rubbers, ceramics, enamels, and glasses. Inorganic colored pigments are based upon crystalline materials comprised of oxides of mixed metals. There are various reasons for using inorganic pigments. Among these are the coloristic properties of the pigments, their visible as well as ultraviolet (UV) and infrared (IR) reflectance characteristics, their light fastness and their high temperature stability. The high temperature stability is necessary when the objects being colored are formed at elevated temperatures.
Pigments exhibit certain colors because they selectively reflect and absorb certain wavelengths of light. White light is an approximately equal mixture of the entire visible spectrum of light. When white light encounters a colored pigment, some wavelengths are absorbed as they interact with the electronic structure of the pigment. These interactions are determined by the chemistry and bonding of the pigment. The wavelengths not absorbed are reflected back to the observer, and this reflected visible light spectrum creates the appearance of a color. For example, ultramarine reflects blue light, typically between 400 and 500 nanometers in wavelength, and absorbs visible light of other wavelengths.
The appearance of pigments is also dependant upon the spectrum of the source light. Sunlight has a high color temperature, and a fairly uniform spectrum, and is considered a standard for white light. Artificial light sources, including fluorescent light, tend to have great peaks in some regions of their spectrum, and deep valleys in other regions. Viewed under these conditions, pigments may exhibit different colors.
A large number complex inorganic color pigments (CICPs) are copper-containing pigments. Examples of copper-containing blacks include C.I. Pigments, Black 26, Black 27, Black 28, and Black 30. All of these blacks have spinel crystal structure. Copper chromite blacks (C.I. Pigments Black 28) are spinels made from copper (II) oxide and chromium (III) oxide green, with a general formula of CuCr2O4. Copper chromites are the most widely used CICP blacks. The copper chromite blacks are general-purpose pigments with durability and temperature resistance. Copper chromite blacks generally provide dark-tone color as well as strong absorption in near IR region. The copper chromite blacks are excellent UV absorbers, and offer good UV opacity to the systems that employ them. Red-colored Cuprous oxide (Cu2O) is commonly used as a pigment, a fungicide, and an antifouling agent for marine paints. The black Cupric oxide is used as a pigment in ceramics to produce blue, red, and green (and sometimes gray, pink, or black) glazes.
According to the definition of CPMA (Color Pigments Manufacturers Association), for the pigments with yellow hue, there are several yellow, buff or brown pigment families. Most of them are titanate compounds doped with transition metals with rutile crystal structure, such as Nickel Antimony Titanium Yellow (Yellow 53), Chrome Antimony Titanium Buff (Brown 24) and Nickel Niobium Titanium Yellow (Yellow 161), etc. To our best knowledge, there is no reported copper-containing yellow pigment. There are also other copper containing pigments, such as Egyptian Blue: a synthetic pigment of calcium copper silicate (CaCuSi4O10), Han Blue (BaCuSi4O10, Paris Green:Copper(II) acetoarsenite (Cu(C2H3O2)2.3Cu(AsO2)2) and Scheele's Green:Copper arsenite CuHAsO3 and some blue and green organic pigments, such as Copper Phthalocyanine and its derivatives.